In order to exploit electromagnetism, materials are commonly used to control and direct the electromagnetic fields. For example, a glass lens in a camera directs the rays of light to form an image, metal cages are used to screen sensitive equipment from electromagnetic radiation, and various forms of ‘black bodies’ are utilised to prevent unwanted reflections. One aspect of electromagnetism which is of particular interest, is the use of materials in the manipulation of electromagnetic waves such as to conceal or cloak objects or volumes of space from detection by an outside observer.
Several known methods exist which attempt to achieve electromagnetic concealment of objects. For example, it is possible to use a series of cameras to project an image to an observer of what he would see if an object in question were not blocking his view path. As a result, the observer does not realise that the object is present. This method, however, relies on the use of active components, and depends heavily on the relative positioning of the object, cameras and observer at all times.
Further known concealment methods include traditional “stealth technology” and the use of low radar cross section structures. These are designed to minimize back reflection of radar or other electromagnetic waves. Whilst these structures can provide a reduced or altered electromagnetic signature, because they involve either the scattering of incident waves away from the target or absorbing incident waves, the objects which they hide are still detectable in transmission.
In their paper, Physics Rev. E Vol 72, Art. No., 016623 (2005), A Alu and N Engheta suggest a scheme for the concealment of spherical and cylindrical objects through the use of plasmonic and metamaterial ‘cloaks’ or covers. Whilst this paper provides a method of reducing the total scattering cross section of such objects, it relies on a specific knowledge of the shape and material properties of the object being hidden. In particular, the electromagnetic cloak and the concealed object form a composite, whose scattering properties can be reduced in the lowest order approximation. Therefore, if the shape of the object changes, the shape of the cloak must change accordingly. Furthermore, this method relies on a resonance effect, such that if the frequency drifts away from its resonant peak, the method is less effective. It is therefore a narrowband method, which cannot be implemented for broadband applications.
A further aspect of electromagnetism which is of interest is the use of materials in electromagnetic sensing and energy harvesting applications. Several known devices exist in this area, such as satellite dishes and solar energy panels. Whilst such prior art devices are operable to collect or detect electromagnetic radiation incident upon them from a number of different directions, and can be moveable to capture radiation incident from any desired direction, they do not have the capability to capture electromagnetic radiation incident from all directions at any given time. Problems therefore arise in applications when the direction of the electromagnetic source is initially unknown or constantly changing, such as in solar energy collection and microwave energy beaming on mobile platforms.